1. Field of the Invention
The present invention relates to a mixing method, a mixing structure, and a micromixer and a microchip having the mixing structure.
2. Description of the Related Art
A μ-TAS (μ-Total Analysis System) has an exceptionally smaller size than that of a conventionally used appliance, i.e., a flask, a test tube and so on. For this reason, amount, cost and disposal of reagent or specimen can be suppressed, and thus an attention is paid to the μ-TAS as for one feature that synthesizing and detection with a very small amount are possible. The μ-TAS can be applied to a clinical analyzing chip, an environmental analyzing chip, a gene analyzing chip (DNA chip), a sanitary analyzing chip, a chemical/biochemical analyzing chip and the like.
For example, U.S. Pat. No. 5,971,158 discloses an extracting apparatus having a flow pass with a width of about 10 μm to about 100 μm. However, it does not disclose a micromixing structure in which a plurality of branched flow passes are arranged three-dimensionally and are interflowed in parallel.
In addition, “The Actualities and Prospects of Microreactor Technique” discloses “LIQUID-SHEET BREAKUP IN MICROMIXERS”. This system is constituted so that a liquid and a gas are allowed to flow on the plane from opposite directions and are allowed to interflow so as to be taken out to the top.
In the world of a micro flow pass where a channel has microscale, both the dimension and the flow rate are small, and the Reynolds number is not more than 200. For example, in the case where water is flowed into an average flow pass with a width of 200 μm to be used in a micro flow pass with a flow rate of 2 mm/s, a Reynolds number becomes 0.4. Therefore, in the world of the micro flow pass (the width of the flow pass is not more than about 500 μm), a laminar flow is dominant unlike a conventional reacting apparatus in which a turbulent flow is dominant.
In the space of microscale, since a specific interface area is large, the laminar flow is advantageous to diffuse mixing in the interface which comes in contact with the laminar flow. The time required for the mixing depends on a cross-sectional area of the interface where two liquids contact and a thickness of a liquid layer.
According to the diffuse theory, since the time (T) required for the mixing is proportional to W2/D wherein the width of the flow pass is W and a diffuse coefficient is D, as the width of the flow pass is made to be smaller, the mixing (diffuse) time becomes faster. Moreover, the diffuse coefficient D is obtained by the following equation:D=Kb×T/6×π×μ×r  (1)(Wherein, T: liquid temperature, μ: viscosity, r: particle radius, Kb: Boltzmann's constant)
For example, a relationship between the width of the flow pass (channel width) and the specific interface area and the diffuse time when particles with diameter of 100 nm (0.1 μm) are used is as shown in FIG. 1.
Namely, in the microscale space, even if mechanical stirring is not used, carrying, reaction and separation of molecules are carried out quickly only by unprompted motion of molecules and particles.
Meanwhile, in a current macroscale apparatus, turbulent mixing according to the mechanical system is generally carried out by using a test tube or the like with diameter of about 5 mm, but the apparent viscosity of a liquid abruptly increases by influences of capillary force and resistance of a flow pass in microscale in comparison with the macroscale, and thus the liquid does not move easily.
For example, In order to compare the mechanical stirring forces required for the mixing in a cylindrical macro flow pass and in a micro flow pass, a model having the following conditions is used:required mechanical stirring force=ΔP×ΔR  (2)(Wherein, ΔP: capillary force, ΔR: resistance of flow pass)ΔP=H×cos θ×τ/A  (3)(Wherein, H: surface tension of liquid, θ: contact angle, τ: outer peripheral length of flow pass section, A: cross-sectional area of flow pass)ΔR=32×μ×L/π×r4  (4)(Wherein, μ: viscosity, L: length of flow pass (axial height), r: radius of flow pass section)
The required mechanical stirring force in the case where a liquid is in the micro flow pass with inner diameter of 0.2 mm up to the height of 0.1 mm is 488281 times as strong as the required mechanical stirring force in the case where a liquid is in the macro flow pass with inner diameter of 5 mm up to the height of 2 mm. Namely, in order to achieve the equivalent mixing in the microscale apparatus by means of the same mechanical stirring as that in the current macroscale apparatus, the stirring force which is about 100000 times is required in the case of the micro flow pass. This is derived from the above calculation of the model.
Therefore, it is considered that the carrying, reaction and separation of molecules can be carried out by positively using diffusion due to unprompted motion of molecules and particles without using the mechanical stirring in the microscale space.
However, when the width of the flow pass is reduced to an extreme in order to quicken the diffuse time efficiently, the resistance of the flow pass becomes extremely large. As a result, the feeding of the liquid cannot be controlled and also very high pressure is required for feeding the liquid. For this reason, the liquid feeding mechanism is enlarged, and thus a microsystem cannot be established entirely. Moreover, when the width of the flow pass is extremely small, an amount of the liquid is extremely small. As a result, the detection limit is lowered and a higher-sensitive detection mechanism is required, and applications are limited in the current direction method.